Products having hydrophilic or hydrophobic surfaces have broad applications running from proton absorption, water transport (e.g. fuel cells), friction control, etc. On a superhydrophobic surface, water stays in a droplet form and rolls off easily when the surface is slightly tilted. Achieving superhydrophobicity usually requires texture on the surface such as the well-known lotus leaves. On a typical flat hydrophobic surface, a water contact angle of as high as 110° can be achieved.
Water, when dropped on a superhydrophilic surface (water contact angle below 20 degree), spreads simultaneously in a radial fashion. If the superhydrophilic surface contains micro-channel features, water wicks into the channels and moves quickly along the channels due to the additional capillary force, with the velocity depending on the channel size. This water wicking behavior has been found particularly useful in facilitating water transport in micro-channels.